1. Field of the Invention
This invention relates to a device for controlling an electromagnetic clutch on a vehicle (hereinafter referred to as "a vehicle electromagnetic clutch control device", when applicable).
2. Prior Art
A vehicle electromagnetic clutch control device has been disclosed, for instance, by Japanese Patent Application (OPI) No. Sho. 63-57342 (the term "OPI" as used herein means an "unexamined published application"). The conventional vehicle electromagnetic clutch control device will be described with reference to FIG. 1.
In FIG. 1, reference numeral 1 designates clutch current control means. The control means 1 includes clutch current calculating means 2, to which travel control data SD and engine control data SE are applied. The clutch current calculating means 2 provides a clutch current digital instruction signal SID according to those data SD and SE, which is applied to a digital-to-analog (D/A) converter 25.
In response to the clutch current digital instruction signal SID, the D/A converter 25 outputs an analog signal, namely, a clutch current analog instruction signal SIA, which is applied to the positive input terminal (+) of a current difference detector 14, to the negative input terminal (-) of which a clutch current feedback signal SF provided by a current detecting amplifier 60 is applied.
Thus, the current difference detector 14 obtains the difference between the clutch current analog instruction signal SIA and the clutch current feedback signal SF, to output a difference signal. The difference signal is applied through a pulse width modulation (PWM) modulator 15 and a resistor 51 to the base of an output transistor 11. The emitter of the transistor 11 is connected to a power supply line, to which the positive terminal of a power source 3 is connected. The negative terminal of the power source 3 is grounded. The collector of the output transistor 11 is grounded through a parallel circuit of a feedback diode 16 and a deexciting resistor 18, and is connected to an output terminal 20.
The output terminal 20 is connected through an electromagnetic clutch 4 to another output terminal 21. The electromagnetic clutch 4 comprises slip rings 42 and 43, and a clutch coil 41. Clutch current is applied through the slip rings 42 and 43 to the clutch coil 41.
The aforementioned clutch current calculating means 2 outputs a clutch release signal SO, which is supplied through a base resistor 27 to the base of a clutch release transistor 28. The emitter of the transistor 28 is grounded, and the collector is Connected to the base of an output transistor 12. The base of the transistor 12 is connected through a base resistor 26 to the power source. The collector of the transistor 12 is connected to the output terminal 21 and is connected through a parallel circuit of an over-voltage preventing diode 17 and a deexciting resistor 19 to the power source. The emitter of the output transistor 12 is grounded through a current detecting resistor 13. The resistor 13 is to detect a clutch current.
One terminal of the current detecting resistor 13, which is connected to the emitter of the output transistor 12, is connected to the positive input terminal (+) of an operational amplifier 61 in the current detecting amplifier 60. The other terminal of the current detecting resistor 13, which is grounded, is connected through a reference resistor 63 to the negative input terminal (-) of the operational amplifier 61, which terminal is connected through a feedback resistor 62 to the output terminal thereof.
The operation of the control device thus organized will be described with reference to a flow chart of FIG. 2.
First, in Step 501, a vehicle speed is calculated by a control unit (not shown). Then, in Step 502, an engine speed (a number of revolutions per minute of an engine) is calculated by the control unit. In Step 503, in response to the vehicle speed and the engine speed, the aforementioned travel control data SO is applied to the clutch current calculating means 2.
Thereafter, Step 504 is effected. In Step 504, the clutch current calculating means 2 receives the engine control data SE. In Step 505, in response to the travel control data SD and the engine control data SE, the clutch current calculating means 2 calculates clutch torque. In Step 506, the clutch current calculating means 2 outputs the clutch current digital signal SID.
The clutch current digital signal SID is applied to the D/A converter 25, where it is converted into an analog signal, namely, the aforementioned clutch current analog signal SIA. The analog signal SIA is applied to the positive input terminal (+) of the current difference detector 14, to the negative input terminal (-) of which the clutch current feedback signal SF provided by the operational amplifier 61 in the current detecting amplifier 60 is applied. That is, the current difference detector 14 obtains the difference between the clutch current analog signal SIA and the clutch current feedback signal SF, to output a difference signal.
The difference signal is applied to the PWM modulator 15. In the PWM modulator 15, the difference signal is subjected to pulse width modulation. The output of the PWM modulator 51 is applied through the resistor 15 to the base of the output transistor 11. Thus, the output transistor 11 is rendered on and off depending on the pulse width with which the difference signal is modulated by the PWM modulator 15. When the output transistor 11 is rendered conductive (on) or non-conductive (off) in this manner, the clutch current is allowed or not allowed to flow to the electromagnetic clutch 4 through the output terminal 20.
On the other hand, during the ordinary clutch engagement, the output transistor 12 is conductive (on). Therefore, when the output transistor 11 is rendered conductive (on) by the difference signal pulse-width-modulated by the PWM modulator, the clutch current flows in a closed loop of the output transistor 11, the output terminal 20, the electromagnetic clutch 4, the output terminal 21, the output transistor 12, the current detecting resistor 13, and ground.
In this operation, the voltage at one terminal of the current detecting resistor 13 is applied to the positive input terminal (+) of the operational amplifier 61 in the current detecting amplifier 60, while the voltage at the other terminal of the current detecting resistor 13 is decreased by the reference resistor 63. The voltage thus decreased is applied, as a reference voltage, to the negative input terminal (-) of the operational amplifier 61. The reference voltage is further applied through the feedback resistor 62 to the output terminal of the operational amplifier 61, and processed with the amplification factor of the latter 61. Thus, the clutch current feedback signal SF is applied from the output terminal of the operational amplifier 61 to the negative input terminal (-) of the current difference detector 14.
Thus, in accordance with the PWM signal, which is provided by pulse-width modulation of the difference signal outputted by the current difference detector 14, the clutch current is allowed to flow in the electromagnetic clutch 4 to activate the latter.
On the other hand, the electromagnetic clutch 4 is released by the clutch release signal SO outputted by the clutch current calculating means 2. The clutch release signal SO is applied through the base resistor 27 to the base of the clutch releasing transistor 28, to render the latter 28 conductive (on). When the clutch releasing transistor 28 is turned on in this way, the collector potential is decreased, so that the output transistor 12 is rendered non-conductive (off). When the output transistor 12 is rendered non-conductive in this manner, the clutch current is interrupted, so that the clutch is released.
The conventional vehicle electromagnetic clutch control device is designed as described above. That is, in detecting the clutch current IC, the current detecting amplifier 60 amplifies the voltage drop by the clutch current flowing in the current detecting resistor 13 connected to the emitter of the output transistor 12.
Hence, the desired clutch current cannot be obtained because of the fluctuation in characteristic of the current detecting resistor 13, the feedback resistor 62, and the reference resistor 63. Therefore, the gain of the operational amplifier 61 is adjusted by replacing the reference resistor 63. However, the operation of the operational amplifier 61 cannot be ensured without the reference resistor 63. Thus, it is rather difficult to control the gain of the operational amplifier 61.
Now, another example of the conventional vehicle electromagnetic clutch control device will be described with reference to FIG. 3.
In FIG. 3, reference numeral 201 designates clutch current control means, which includes clutch current calculating means 202. The latter 202 has an interface 223 and a computer 224.
In the clutch current calculating means 202, travel control data SD and engine control data SE are applied through the interface 223 to the computer 224. The computer 224 forms a clutch current digital instruction signal SID according to the travel control data SD and the engine control data SE. The signal SID is applied to a D/A (digital-to-analog) converter 225, where it is converted into an analog signal, namely, a clutch current analog instruction signal SIA, which is applied to the positive input terminal (+) of a current difference detector 214, to the negative input terminal (-) of which a clutch current feedback signal SF provided at one terminal of current detecting means, namely, a current detecting resistor 213 is applied.
Thus, the current difference detector 214 obtains the difference between the clutch current analog instruction signal SIA and the clutch current feedback signal SF, to output a difference signal. The difference signal is applied through a PWM (pulse width modulation) modulator 215 and a resistor 251 to the base of an output transistor 211.
The emitter of the output transistor 211 is connected to a power supply line, to which the positive terminal of a power source 203 is connected. The negative terminal of the power source 203 is grounded. The collector of the output transistor 211 is grounded through a feedback diode 216, which is shunted by a series circuit of a deexciting resistor 218 and a deexciting current control transistor 260. The collector of the output transistor 211 is connected to an output terminal 220.
The output terminal 220 is connected through an electro-magnetic clutch 204 to another output terminal 221. The electromagnetic clutch 204 comprises slip springs 242 and 243, and a clutch coil 241. Clutch current is supplied through the slip rings 242 to the clutch coil 241.
The aforementioned clutch current calculating means 202 outputs a clutch release signal SO, which is applied through a resistor 272 to the base of a base control transistor 271. The collector of the transistor 271 is connected through a base resistor 252 to the base of an output transistor 212, and the emitter of the transistor 271 is connected to the power source.
The collector of the output transistor 212 is connected to the output terminal 221, and connected through an over-voltage preventing quick-break diode 217 to the power source, and connected through a deexciting resistor 219 to the collector of a deexciting current control transistor 261. The emitter of the output transistor 212 is grounded through the aforementioned current detecting resistor 213. The current detecting resistor 213 is used to detect a clutch current.
The above-described clutch current calculating means 202 supplies deexciting instruction signals through resistors 262 and 263 to the deexciting current control transistors 260 and 261, respectively. The emitter of the transistor 260 is grounded. The emitter of the transistor 261 is connected to the power source.
The operation of the vehicle electromagnetic clutch control device thus organized will be described.
A control unit (not shown) calculates a vehicle speed, and an engine speed (a number of revolutions per minute of an engine). In accordance with the vehicle speed and the engine speed, the aforementioned travel control data SD is applied to the clutch current calculating means 202.
The clutch current calculating means 202 further receives the engine control data SE. In response to the travel control data SD and the engine control data SE, the clutch current calculating means 202 calculates clutch torque to output the clutch current digital instruction signal SID.
The clutch current digital instruction signal SID is applied to the D/A converter 225, where it is converted into the clutch analog instruction signal SIA. The signal SIA is applied to the positive input terminal (+) of the current difference detector 214, to the negative input terminal (-) of which the clutch current feedback signal SF provided at the one terminal of the current detecting resistor 213 is applied. Thus, the current difference detector 214 outputs the difference signal between the clutch current analog instruction signal SIA and the clutch current feedback signal SF.
The difference signal is applied to the PWM modulator 215, where it is subjected to pulse width modulation (PWM). The output of the PWM modulator is applied through the resistor 251 to the base of the output transistor 211. That is, the output transistor 211 is rendered on and off depending on the pulse width with which the difference signal is modulated by the PWM modulator 215. When the output transistor 11 is rendered conductive (on) or non-conductive (off) in this manner, the clutch current is allowed or not allowed to flow to the electromagnetic clutch 204 through the output terminal 220. At the engagement of the electromagnetic clutch 204, both the deexciting current control transistors 260 and 261 are rendered non-conductive (off).
On the other hand, during the ordinary clutch engagement, the output transistor 212 is rendered conductive (on). For this purpose, the clutch current calculating means 202 outputs the clutch current release signal SO to render the base control transistor 271 conductive (on), so that the output transistor 212 is rendered conductive (on). Thus, when the output transistor 211 is turned on by the difference signal pulse-modulated by the PWM modulator 215, the clutch current flows in a closed loop of the output transistor 211, the output terminal 220, the electromagnetic clutch 204, the output terminal 221, the output transistor 212, the current detecting resistor 213, and ground.
In this operation, the clutch current feedback signal SF provided at the one terminal of the current detecting resistor 213 is applied to the negative input terminal (-) of the current difference detector 214.
Thus, in accordance with the PWM signal, which is provided by pulse-width modulation of the difference signal outputted by the current difference detector 214, the clutch current is allowed to flow in the electromagnetic clutch 204 to activate the latter.
On the other hand, the electromagnetic clutch 4 is released by the clutch release signal SO outputted by the clutch current calculating means 202. The output transistor 212 is normally kept rendered conductive (on), and the release of the clutch is controlled by the clutch release signal SO. The clutch release signal SO is outputted by the clutch current calculating means 202, and applied through the base resistor 272 to the base of the base control transistor 271, to render the latter 271 conductive (on). When the base control transistor 271 is turned on in this way, the collector potential is decreased, and the output transistor 212 is therefore turned off. When the output transistor 212 is rendered non-conductive in this manner, the clutch current is interrupted, so that the clutch is released.
When both the transistors 211 and 212 are rendered non-conductive (off), a deexciting current is allowed to flow in the direction opposite to the direction of the clutch current IC, to release the clutch. For this purpose, the clutch current calculating means 202 outputs a deexciting instruction signal, which is applied through the resistors 262 and 263 to the deexciting current control transistors 260 and 261, respectively. As a result, both the transistors 260 and 261 are rendered conductive (on), so that the deexciting current is supplied from the power source through the deexciting resistors 218 and 219 to the electromagnetic clutch 204 in the direction opposite to the direction of the clutch current IC. Thus, when the clutch is released, its residual torque can be corrected.
The conventional vehicle electromagnetic clutch control device is designed as described above. In order to detect the clutch current IC, the voltage drop by the clutch current flowing in the current detecting resistor 213 connected to the emitter of the output transistor 212 is detected.
Hence, when the output transistor 212 is conductive (on), the base current IB of the output transistor 212 flows in the current detecting resistor 213. As a result, the detected current is deviated in value from the true clutch current. That is, the conventional vehicle electromagnetic clutch control device suffers from problems that the clutch current is not accurately controlled, and the control of small torque is not satisfactorily carried out.